Vinylpyrrolidone (VP)/vinyl acetate (VA) copolymers, which can be supplied in the form of a powder or solutions, are much in demand for use in the pharmaceutical, cosmetic or related arts which require little or no toxicity. These copolymers are also useful as adhesives and abrasion resistant coatings having a glossy, colorless appearance and wherein the degree of water solubility can be varied with the concentration of VP monomer and the homogeneous distribution of VP in the copolymeric product.
Several methods of copolymerization have been employed to satisfy the many uses of this copolymer which, theoretically, can contain from 20 to 80% VP concentration. one process is described in U.S. Pat. No. 2,667,473 which is carried out using benzene solvent in the presence of hydrogen peroxide or azobis(isobutyramide) free radical initiator. This process encounters several difficulties involving the handling of the initiator which is highly insoluble. Furthermore, both initiator and solvent are toxic so that extreme care is required for product purification. Replacement of azobis(isobutyramide), referred to herein as AIBN, with non-toxic t-butylperoxy pivalate and benzene with aqueous isopropanol has been described in U.S. Pat. No. 4,554,311. However, this process involves a large initial charge of monomers to the reactor prior to contact with initiator; and since VP monomer is significantly more reactive than VA, initial stages of the polymerization produce chains rich in VP and, only in the later polymerization stages, is the VP/VA copolymeric portion of the heterogeneous product formed, resulting in a water insoluble product. Homogeneity in the copolymeric structure, i.e. the copolymer having a uniform distribution of VP and VA monomer units in its macromolecular chain structure, is highly desirable from the standpoint of water solubility.
British Pat. No. 1,323,028 describes a homogeneous VP/VA copolymer prepared by an intricate process employing a continuous phase in which VP is soluble and a disperse phase in which VA is suspended. Copolymerization takes place exclusively in the disperse phase while the continuous phase functions as a VP reservoir. Although successful in producing a homogeneous product, this process, using AIBN initiator is extremely limited in the choice of solvents for the disperse phase, e.g. benzene, toluene and for the continuous phase, e.g. acids or polyols, and is further complicated by the need for the removal of both solvent and substantial portions of VP monomer in the continuous phase for recovery of product. Additionally, this process employing the VA reservoir, fails to maximize the reactive efficiency of the VA monomer.
Accordingly, it is an object of the present invention to overcome the above problems and to produce a substantially homogeneous VP/VA copolymer over a broad range of VP concentrations and polymeric K values of between about 10 to about 100.
Another object is to provide a process for the preparation of a substantially homogeneous VP/VA copolymers by a simplified and economical process which is readily adaptable to commercial use.
Still another object is to provide a copolymerization process for inline control of VP concentration in the VP/VA copolymeric macromolecular product.
These and other objects of the invention will become apparent from the following description and disclosure.